Echo cancelling device, communication device, and echo cancelling method having the error signal generating circuit

ABSTRACT

An echo cancelling device includes an adaptive filter which synthesizes a spurious echo signal from a receiving signal, a first subtractor which generates an echo cancellation signal based on an input signal including a voice signal of a speaker and the spurious echo signal, and an error signal generating circuit which generates an error signal based on the input signal and the echo cancellation signal.

The present application is a Continuation Application of U.S. patentapplication Ser. No. 12/461,329, filed on Aug. 7, 2009, which is basedon and claims priority from Japanese patent application No. 2008-243702,filed on Sep. 24, 2008, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an echo cancelling technology used in acommunication device. In particular, the present invention relates tothe echo cancelling technology in a double talk state.

2. Description of the Related Art

Recently, mobile phones having a videophone function have come on themarket, and the number of users who talk in a hands free talk mode hasincreased upon using the videophone function. In the hands free talkmode, a sound output level from a speaker may be so high that a soundfrom the speaker may be received by a microphone. This phenomenon iscalled as “sneaking”.

As illustrated in FIG. 1, when the sneaking occurs on a far end talkerside, a phenomenon that a voice of a near end talker is heard from aspeaker on a near end talker side after a while occurs (that is calledan “echo”). Such an echo is a phenomenon unpleasant to the near endtalker. In addition, as illustrated in FIG. 2, when the sneaking occurson both the far end talker side and the near end talker side, anacoustic closed loop may be formed. As a loop gain increases, anoscillation may be generated so that a phenomenon of generating a largesound like “boom” (so-called “howling sound”) will occur. The howlingsound is also an unpleasant phenomenon, and the user on both ends willhave no other choice but to stop talking. In order to suppress such anecho or a howling sound as described above, an echo canceller is used.

FIG. 3 illustrates a structure of the echo canceller described in“Technology of Digital Audio” written and edited by Nobuhiko Kitawaki,issued by the Telecommunications Association, distributed by Ohmsha,Ltd. ISBN4-88549-905-4. A signal transmitted from a mobile terminal onthe near end talker side to a mobile terminal on the far end talker sideis a transmitting signal e(k). On the contrary, a signal received by themobile terminal on the near end talker side from the mobile terminal onthe far end talker side is a receiving signal x(k). The receiving signalx(k) is delivered from a speaker of the mobile terminal on the near endtalker side. In addition, it is supposed that the near end talker isperforming a hands free talk. Therefore, an echo signal y(k) isgenerated by sneaking of the receiving signal x(k) delivered from thespeaker, and is received by a microphone of the mobile terminal on thenear end talker side. This echo signal y(k) is expressed by Equation(1).

y(k)=h(k)×x(k)   Equation (1)

The parameter h(k) in Equation (1) is a conversion coefficient from thereceiving signal x(k) into the echo signal y(k). In other words, theconversion coefficient h(k) indicates a transmission characteristic ofan acoustic echo path from the speaker to the microphone, which dependson an environment in which the mobile terminal on the near end talkerside is placed. In addition to the echo signal y(k) described above, avoice signal v(k) of the near end talker and an ambient noise signaln(k) are also received by the microphone of the mobile terminal on thenear end talker side. In other words, an input signal yin(k) received bythe microphone of the mobile terminal on the near end talker side isexpressed by Equation (2).

yin(k)=v(k)+n(k)+y(k)   Equation (2)

Note that k indicates time as to the parameters described above. Thesame is true in the following description.

The echo canceller illustrated in FIG. 3 includes an adaptive filter anda subtractor so as to cancel the echo signal y(k). First, the adaptivefilter synthesizes a spurious echo signal y′(k) from the receivingsignal x(k) based on the NLMS algorithm. This spurious echo signal y′(k)is an echo signal estimated by the adaptive filter and is expressed byEquation (3).

y′(k)=h′(k)×x(k)   Equation (3)

The parameter k′(k) in Equation (3) is a conversion coefficient from thereceiving signal x(k) into the spurious echo signal y′(k). In otherwords, the conversion coefficient h′(k) indicates a transmissioncharacteristic of the acoustic echo path from the speaker to themicrophone, which is estimated by the adaptive filter. The adaptivefilter delivers the obtained spurious echo signal y′(k) to thesubtractor.

The subtractor receives the input signal yin(k) from the microphone.Then, the subtractor generates the transmitting signal e(k) bysubtracting the above-mentioned spurious echo signal y′(k) from thereceived input signal yin(k). The transmitting signal e(k) generated bythe subtractor is expressed by Equation (4).

$\begin{matrix}\begin{matrix}{{e(k)} = {{{yin}(k)} - {y^{\prime}(k)}}} \\{= {{v(k)} + {n(k)} + {y(k)} - {y^{\prime}(k)}}}\end{matrix} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

The adaptive filter illustrated in FIG. 3 performs feedback controlbased on the transmitting signal e(k). Specifically, the adaptive filterupdates the above-mentioned conversion coefficient h′(k) so that thetransmitting signal e(k) becomes zero. Here, it is supposed that thenear end talker is not talking so that the voice signal v(k) of the nearend talker is zero. In addition, it is supposed that a level of theambient noise signal n(k) can be neglected. In this case, thetransmitting signal e(k) generated by the subtractor is expressed byEquation (5).

e(k)=y(k)−y′(k)   Equation (5)

The adaptive filter updates the above-mentioned conversion coefficienth′(k) so that the transmitting signal e(k) expressed by Equation (5)becomes zero. In other words, the adaptive filter estimates thetransmission characteristic h(k) of the acoustic echo path from thespeaker to the microphone so that the echo signal y(k) received by themicrophone is cancelled. The transmitting signal e(k) expressed byEquation (5) is an estimated error, and it can be said that the adaptivefilter performs the feedback control so that the estimated error e(k)becomes zero. When the conversion coefficient h′(k) of the adaptivefilter matches the transmission characteristic h(k) of the acoustic echopath, the spurious echo signal y′(k) agrees with the actual echo signaly(k), whereby echo cancellation is normally performed.

In this way, if the near end talker is not talking but only the far endtalker is talking, the echo cancellation is normally performed.Actually, however, there often occurs the situation in which not onlythe far end talker but also the near end talker is talkingsimultaneously (hereinafter referred to as “double talk state”). In thedouble talk state, the transmitting signal e(k) generated by thesubtractor is expressed by Equation (4). Even if the ambient noisesignal n(k) can be neglected, the voice signal v(k) of the near endtalker cannot be neglected. In this case, the adaptive filter performsthe feedback control so that the transmitting signal e(k) expressed byEquation (4) becomes zero, and hence it is impossible to remove only theecho signal y(k) normally. In other words, the adaptive filtermisestimates the transmission characteristic h(k) due to a disturbanceother than the echo signal y(k) received by the microphone, with theresult that the performance of the echo cancellation is deterioratedsignificantly.

As described above, the echo canceller illustrated in FIG. 3 becomesunstable with respect to a disturbance, and particularly in the doubletalk state, the performance of the echo cancellation is deterioratedsignificantly. A related technology for a purpose of solving theabove-mentioned problem is described in Japanese Patent ApplicationLaid-open No. 2002-76999.

FIG. 4 illustrates a structure of the echo canceller described inJapanese Patent Application Laid-open No. 2002-76999. The echo cancellerillustrated in FIG. 4 includes a power estimation circuit 103, a stepsize decision circuit 104, a noise level estimation circuit 106, and anear end voice level estimation circuit 107 in addition to an adaptivefilter 101 and a subtractor 102.

Similarly to the case of FIG. 3, the adaptive filter 101 synthesizes thespurious echo signal y′(k) from the receiving signal x(k), and thesubtractor 102 subtracts the spurious echo signal y′(k) from the inputsignal yin(k) so as to generate the transmitting signal e(k). Thetransmitting signal e(k) is the same as that expressed by Equation (4).On the other hand, the power estimation circuit 103 estimates power ofthe receiving signal x(k) based on the receiving signal x(k) from thefar end talker. In addition, the noise level estimation circuit 106 andthe near end voice level estimation circuit 107 respectively estimatelevels of the ambient noise signal n(k) and the voice signal v(k) basedon the transmitting signal e(k).

The step size decision circuit 104 decides a step size based on theestimated power of the receiving signal x(k), the estimated level of theambient noise signal n(k) and the estimated level of the voice signalv(k). The step size means an update quantity of the conversioncoefficient h′(k) in the adaptive filter 101. For instance, when theestimated level of the voice signal v(k) or the ambient noise signaln(k) is relatively small, i.e., when it is determined that the inputsignal yin(k) received by the microphone is mainly the echo signal y(k),the step size decision circuit 104 sets the step size to be relativelylarge. On the other hand, when the estimated level of the voice signalv(k) or the ambient noise signal n(k) is relatively large, i.e., when itis determined that the disturbance received by the microphone is large,the step size decision circuit 104 sets the step size to be relativelysmall. The step size (update quantity) decided in this way is suppliedto the adaptive filter 101 together with the transmitting signal e(k).

The adaptive filter 101 updates the conversion coefficient h′(k) so thatthe transmitting signal e(k) becomes zero. On this occasion, theadaptive filter 101 updates the conversion coefficient h′(k) inaccordance with the step size decided by the step size decision circuit104. In other words, when the estimated level of the voice signal v(k)or the ambient noise signal n(k) is relatively small, the adaptivefilter 101 updates the conversion coefficient h′(k) by the large step.On the other hand, when the estimated level of the voice signal v(k) orthe ambient noise signal n(k) is relatively large, the adaptive filter101 updates the conversion coefficient h′(k) by the small step.

In this way, the echo canceller illustrated in FIG. 4 estimates thelevel of the voice signal v(k) and the ambient noise signal n(k) whichare contained in the input signal yin(k), and sets the update quantityof the conversion coefficient h′(k) to be variable in accordance withthe situation. Thus, it is possible to suppress the transmissioncharacteristic h(k) from being largely misestimated by the adaptivefilter 101, whereby the stability with respect to the disturbance isimproved. In other words, it is possible to suppress the echocancellation performance from being significantly deteriorated.

Japanese Patent Application Laid-open No. 2008-141734 discloses an echocanceller that is used for a loudspeaker call system for performing aloudspeaker call using a speaker and a microphone. The echo cancellerincludes an adaptive filter portion and an echo suppressing portion. Theadaptive filter portion identifies an impulse response of a feedbackpath constituted of an acoustic connection between the speaker and themicrophone, in an adaptive manner, and estimates an echo component ofthe feedback path based on an input signal supplied to the feedbackpath. Further, the adaptive filter portion subtracts the estimated echocomponent from a microphone input signal supplied from the feedbackpath. The echo suppressing portion performs an echo suppressing processon an echo cancellation output signal delivered from the adaptive filterportion. Specifically, the echo suppressing portion determines an echosuppressing quantity based on a Wiener filtering method by using an echoreducing quantity that is defined based on a ratio between theabove-mentioned microphone input signal and a voice signal on the nearend side which mixes in the feedback path. Then, the echo suppressingportion multiplies the echo suppressing quantity and the echocancellation output signal delivered from the adaptive filter portiontogether.

In the echo canceller illustrated in FIG. 4, the step size is decidedbased on three parameters including the estimated power of the receivingsignal x(k), the estimated level of the ambient noise signal n(k), andthe estimated level of the voice signal v(k). However, it is difficultto decide an appropriate step size by such a method as described above.It is because that the power of the receiving signal x(k), the levels ofthe ambient noise signal n(k), and the voice signal v(k) vary largely inaccordance with a telephone environment.

SUMMARY

According to a first aspect of the present invention, an echo cancellingdevice is provided. The echo cancelling device comprises: an adaptivefilter; a subtractor; and an error signal generating circuit. Theadaptive filter synthesizes a spurious echo signal from a receivingsignal before being delivered from a speaker. The subtractor subtractsthe spurious echo signal from an input signal received by a microphoneso as to generate an echo cancellation signal. The error signalgenerating circuit generates an error signal by removing a spuriousvoice signal corresponding to a voice signal of a talker from the echocancellation signal. The adaptive filter updates a characteristic of theadaptive filter so that an amplitude of the error signal becomessmaller.

According to a second aspect of the present invention, a communicationdevice is provided. The communication device comprises: a microphonewhich receives an input signal; a speaker which delivers a receivingsignal; and an echo cancelling device. The echo cancelling deviceincludes: an adaptive filter which synthesizes a spurious echo signalfrom the receiving signal before being delivered from the speaker; asubtractor which subtracts the spurious echo signal from the inputsignal so as to generate an echo cancellation signal; and an errorsignal generating circuit which generates an error signal by removing aspurious voice signal corresponding to a voice signal of a talker fromthe echo cancellation signal. The adaptive filter updates acharacteristic of the adaptive filter so that an amplitude of the errorsignal becomes smaller.

According to a third aspect of the present invention, an echo cancellingmethod is provided. The echo cancelling method comprises: (A)synthesizing, by means of an adaptive filter, a spurious echo signalfrom a receiving signal before being delivered from a speaker; (B)subtracting the spurious echo signal from an input signal received by amicrophone so as to generate an echo cancellation signal; (C) generatingan error signal by removing a spurious voice signal corresponding to avoice signal of a talker from the echo cancellation signal; and (D)updating a characteristic of the adaptive filter so that an amplitude ofthe error signal becomes smaller.

According to the echo cancelling technology of the present invention,stability of the adaptive filter is enhanced with respect todisturbances except for the echo signal received by the microphone. As aresult, an excellent echo cancellation performance can be obtained in adouble talk state as well.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a conceptual diagram illustrating a principle of generation ofan echo;

FIG. 2 is a conceptual diagram illustrating a howling sound;

FIG. 3 is a block diagram illustrating a structure of an echo cancelleraccording to a related technology;

FIG. 4 is a block diagram illustrating a structure of an echo cancelleraccording to another related technology;

FIG. 5 is a schematic diagram illustrating a structure of a mobileterminal according to an embodiment of the present invention;

FIG. 6 is a block diagram illustrating a structure of an echo canceller(echo cancelling device) according to the embodiment of the presentinvention; and

FIG. 7 is a flowchart illustrating an operation of the echo cancelleraccording to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, with reference to the attached drawings, an echo cancellingtechnology according to an embodiment of the present invention isdescribed. The echo cancelling technology according to this embodimentcan be applied to a communication device such as a mobile phone, amobile terminal, a fixed telephone, a communication conference terminal,or the like.

1. Structures

FIG. 5 schematically illustrates a mobile terminal 1 as an example ofthe communication device to which the echo cancelling technology of thisembodiment is applied. The mobile terminal 1 includes a microphone 2(input portion), a speaker 3 (output portion), an antenna 4, and adisplay 5. The mobile terminal 1 receives speech data from anothermobile terminal via the antenna 4 and transmits speech data to theanother mobile terminal. The mobile terminal 1 illustrated in FIG. 5 hasa videophone function, for example, and a user (near end talker) canperform hands free talk by using the mobile terminal 1. In the handsfree talk mode, the display 5 displays an image of a far end talker andthe like, and the speaker 3 delivers a voice of the far end talker. Avoice of the near end talker, ambient noise, an echo, and the like arereceived by the microphone 2.

The mobile terminal 1 of this embodiment is further equipped with anecho canceller (echo cancelling device) 10. FIG. 6 is a block diagramillustrating a structure of the echo canceller 10 according to thisembodiment. The echo canceller 10 of this embodiment includes anadaptive filter 20, a first subtractor 30, and an error signalgenerating circuit 40. The error signal generating circuit 40 includes aresidual echo suppressor 41, a noise suppressor 42, and a secondsubtractor 43.

A signal transmitted from the mobile terminal 1 via the antenna 4 to themobile terminal on the far end talker side is a transmitting signale(k). On the other hand, a signal received by the mobile terminal 1 viathe antenna 4 from the mobile terminal on the far end talker side is areceiving signal x(k). The receiving signal x(k) is delivered from thespeaker 3 of the mobile terminal 1. In addition, it is supposed that thenear end talker is performing the hands free talk. Therefore, an echosignal y(k) generated by sneaking of the receiving signal x(k) deliveredfrom the speaker 3 is received by the microphone 2 of the mobileterminal 1. This echo signal y(k) is expressed by Equation (6).

y(k)=h(k)×x(k)   Equation (6)

The parameter h(k) in Equation (6) is a conversion coefficient from thereceiving signal x(k) to the echo signal y(k). In other words, theconversion coefficient h(k) indicates a transmission characteristic ofan acoustic echo path from the speaker 3 to the microphone 2, whichdepends on an environment in which the mobile terminal 1 is placed. Inaddition to the above-mentioned echo signal y(k), a voice signal v(k) ofthe near end talker and an ambient noise signal n(k) are also receivedby the microphone 2 of the mobile terminal 1. In other words, an inputsignal yin(k) received by the microphone 2 of the mobile terminal 1 isexpressed by Equation (7).

yin(k)=v(k)+n(k)+y(k)   Equation (7)

Note that k denotes time as to the parameters described above. The sameis true in the following description.

2. Operations

FIG. 7 is a flowchart illustrating an operation of the echo canceller 10according to this embodiment. Hereinafter, with reference to FIGS. 6 and7, the operation of the echo canceller 10 according to this embodimentis described in detail.

(Step S1)

First, the adaptive filter 20 synthesizes a spurious echo signal y′(k)based on the NLMS algorithm from the receiving signal x(k) before beingdelivered from the speaker 3. This spurious echo signal y′(k) is an echosignal estimated by the adaptive filter 20 and is expressed by Equation(8).

y′(k)=h′(k)×x(k)   Equation (8)

The parameter h′(k) in Equation (8) is a conversion coefficient from thereceiving signal x(k) to the spurious echo signal y′(k). In other words,the conversion coefficient h′(k) indicates a transmission characteristicof the acoustic echo path from the speaker 3 to the microphone 2, whichis estimated by the adaptive filter 20. The adaptive filter 20 deliversthe obtained spurious echo signal y′(k) to the first subtractor 30.

(Step S2)

The first subtractor 30 receives the input signal yin(k) that issupplied from the microphone 2. Then, the first subtractor 30 subtractsthe above-mentioned spurious echo signal y′(k) from the received inputsignal yin(k) so as to generate an echo cancellation signal p(k). Theecho cancellation signal p(k) generated by the first subtractor 30 isexpressed by Equation (9).

$\begin{matrix}\begin{matrix}{{p(k)} = {{{yin}(k)} - {y^{\prime}(k)}}} \\{= {{v(k)} + {n(k)} + \left( {{y(k)} - {y^{\prime}(k)}} \right)}}\end{matrix} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

The parameter “y(k)−y′(k)” in Equation (9) indicates an error of echoestimation performed in the adaptive filter 20 and is referred to as a“residual echo signal” in the following description.

(Step S3)

The residual echo suppressor 41 receives the echo cancellation signalp(k) generated by the first subtractor 30 and the input signal yin(k)supplied from the microphone 2. Then, the residual echo suppressor 41removes the residual echo signal “y(k)−y′(k)” from the echo cancellationsignal p(k), so as to generate a residual echo cancellation signal q(k).The removal of the residual echo signal “y(k)−y′(k)” can be performed byusing the method described in Japanese Patent Application Laid-open No.2008-141734. The residual echo cancellation signal q(k) generated by theresidual echo suppressor 41 is expressed by Equation (10).

$\begin{matrix}\begin{matrix}{{q(k)} = {{p(k)} - \left( {{y(k)} - {y^{\prime}(k)}} \right)}} \\{= {{v(k)} + {n(k)}}}\end{matrix} & {{Equation}\mspace{14mu} (10)}\end{matrix}$

(Step S4)

The noise suppressor 42 receives the residual echo cancellation signalq(k) generated by the residual echo suppressor 41. Then, the noisesuppressor 42 removes a noise signal that is equal to the ambient noisesignal n(k) from the residual echo cancellation signal q(k). As thenoise suppressor 42 of this embodiment, it is possible to use the onedescribed in the paper: “Noise Suppression with High Speech QualityBased on Weighted Noise Estimation and MMSE STSA”, KATO Masanori,SUGIYAMA Akihiko, SERIZAWA Masahiro, Institute of Electronics,Information and Communication Engineers, Papers VOL. J87-A, No. 7, pp.851-860, July 2004.

As described in the above-mentioned paper, the noise suppressor 42according to this embodiment removes a noise component based on theweighted noise estimation and the MMSE STSA method. More specifically,the noise suppressor 42 uses a degraded voice weighted in accordancewith an estimated value of a voice-to-noise ratio so as to update anoise estimated value continuously. The noise estimated value is storedin a predetermined storage unit. Then, the noise suppressor 42 subtractsa noise estimated value n′(k−1) that is equal to the ambient noisesignal n(k) from a residual echo cancellation signal q(k−1) of theprevious sampling period so as to generate a spurious voice signalr(k−1) expressed by Equation (11).

$\begin{matrix}\begin{matrix}{{r\left( {k - 1} \right)} = {{q\left( {k - 1} \right)} - {n^{\prime}\left( {k - 1} \right)}}} \\{= {{v\left( {k - 1} \right)} + {n\left( {k - 1} \right)} - {n^{\prime}\left( {k - 1} \right)}}}\end{matrix} & {{Equation}\mspace{14mu} (11)}\end{matrix}$

As described above, the spurious echo signal y′(k) is removed in StepS2, the residual echo signal “y(k)−y′(k)” is removed in Step S3, and thenoise component is removed in Step S4. Therefore, the signal r(k−1)generated by the noise suppressor 42 is the spurious voice signalcorresponding to the voice signal v(k) of the near end talker. In otherwords, the noise suppressor 42 removes the noise component from theresidual echo cancellation signal q(k) so as to extract the spuriousvoice signal r(k−1). The noise suppressor 42 delivers the obtainedspurious voice signal r(k−1) as a transmitting signal e(k−1). Thetransmitting signal e(k−1) becomes the spurious voice signal r(k−1) thatis almost the same as the voice signal v(k−1), and hence speech qualitycan be improved. Further, the noise suppressor 42 delivers the obtainedspurious voice signal r(k−1) to the second subtractor 43.

(Step S5)

The second subtractor 43 receives the echo cancellation signal p(k)generated by the first subtractor 30. In addition, the second subtractor43 receives the spurious voice signal r(k−1) generated by the noisesuppressor 42. Then, the second subtractor 43 subtracts the spuriousvoice signal r(k−1) from the echo cancellation signal p(k) so as togenerate a signal s(k−1) expressed by Equation (12).

$\begin{matrix}\begin{matrix}{{s\left( {k - 1} \right)} = {{p(k)} - {r\left( {k - 1} \right)}}} \\{= {\left( {{v(k)} - {r\left( {k - 1} \right)}} \right) + {n(k)} + \left( {{y(k)} - {y^{\prime}(k)}} \right)}}\end{matrix} & {{Equation}\mspace{14mu} (12)}\end{matrix}$

In this embodiment, it is supposed that the ambient noise signal n(k)has a level that can be neglected. In addition, as described above, thespurious voice signal r(k−1) has a level corresponding to the voicesignal v(k) of the near end talker. Therefore, it can be said that thesignal s(k−1) generated by the second subtractor 43 indicates a level ofthe residual echo “y(k)−y′(k)” as a result of the echo cancellationprocess. In other words, the signal s(k−1) indicates the error of echoestimation in the adaptive filter 20. In this sense, the signal s(k−1)generated by the second subtractor 43 is referred to as an “errorsignal” in the following description.

The residual echo suppressor 41, the noise suppressor 42 and the secondsubtractor 43 described above constitute the error signal generatingcircuit 40 for generating the error signal s(k−1). The error signalgenerating circuit 40 removes the spurious voice signal r(k−1)corresponding to the voice signal v(k) of the talker from the echocancellation signal p(k) so as to generate the error signal s(k−1).Then, the error signal s(k−1) generated by the error signal generatingcircuit 40 is delivered to the adaptive filter 20. In other words,according to this embodiment, not the transmitting signal but the errorsignal s(k−1) is fed back to the adaptive filter 20.

(Step S6)

The adaptive filter 20 receives the error signal s(k−1) and updates itscharacteristic so that an amplitude of the error signal s(k−1) becomessmaller. In other words, the adaptive filter 20 updates the conversioncoefficient h′(k) so that the error signal s(k−1) becomes as small aspossible. This process corresponds to the process of estimating thetransmission characteristic h(k) of the acoustic echo path from thespeaker 3 to the microphone 2 so that the echo signal y(k) received bythe microphone 2 is cancelled. As described above, the error signals(k−1) indicates an error in the estimation process, and it can be saidthat the adaptive filter 20 performs the feedback control so that theestimated error s(k−1) becomes zero. When the conversion coefficienth′(k) of the adaptive filter 20 matches the transmission characteristich(k) of the acoustic echo path, the spurious echo signal y′(k) becomesequal to the actual echo signal y(k) so that the echo can be completelycancelled.

(Step S7)

Steps S1 to S6 described above are repeated until the process for thedata is finished.

3. Effects

As described above, according to this embodiment, the adaptive filter 20performs the feedback control based on the error signal s(k−1). Theerror signal s(k−1) does not contain a component corresponding to thevoice signal v(k). It is because that the error signal generatingcircuit 40 removes the spurious voice signal r(k−1) corresponding to thevoice signal v(k) from the echo cancellation signal p(k) so as togenerate the error signal s(k−1).

The component corresponding to the voice signal v(k) is eliminated fromthe error signal s(k−1), and hence stability with respect to thedisturbance is enhanced in the adaptive filter 20 that performs thefeedback control based on the error signal s(k−1). In other words, it ispossible to prevent the update of the characteristic of the adaptivefilter 20 (conversion coefficient h′(k)) from being unstable also in thedouble talk state in which the voice signal v(k) is received. In otherwords, also in the double talk state, the performance of the echocanceller 10 is not lowered, whereby a good echo cancellationperformance can be realized.

In addition, unlike the echo canceller illustrated in FIG. 4, it is notnecessary to determine the step size based on three parameters includingthe estimated power of the receiving signal x(k), the estimated level ofthe ambient noise signal n(k), and the estimated level of the voicesignal v(k). It is possible to realize a good echo cancellationperformance with a simple structure.

In this way, the embodiment of the present invention has been describedwith reference to the attached drawings.

It is apparent that the present invention is no limited to the aboveembodiments, but may be modified and changed without departing from thescope and spirit of the invention.

1. An echo cancelling device, comprising: an adaptive filter whichsynthesizes a spurious echo signal from a receiving signal; a firstsubtractor which generates an echo cancellation signal based on an inputsignal including a voice signal of a speaker and the spurious echosignal; and an error signal generating circuit which generates an errorsignal based on the input signal and said echo cancellation signal. 2.An echo cancelling device according to claim 1, wherein said errorsignal generating circuit includes: a residual echo suppressor whichgenerates a residual echo cancellation signal by removing a differencebetween said echo signal and said spurious echo signal from said echocancellation signal; a noise suppressor which generates the spuriousvoice signal based on the residual echo cancellation signal; and asecond subtractor which generates the error signal by subtracting thespurious voice signal from said echo cancellation signal.
 3. An echocancelling device according to claim 1, wherein the error signalgenerating circuit generates the error signal by removing the spuriousecho signal from an echo signal.
 4. An echo cancelling device accordingto claim 1, wherein the adaptive filter updates a characteristic of theadaptive filter such that an amplitude of said error signal becomessmaller.
 5. An echo cancelling device according to claim 1, wherein theerror signal is unaffected by the voice signal of the speaker.
 6. Anecho cancelling device, comprising: an adaptive filter which synthesizesa spurious echo signal from a receiving signal; a first subtractor whichgenerates an echo cancellation signal based on an input signal and thespurious echo signal; and a second subtractor which generates an errorsignal based on said echo cancellation signal.